Note that the split comand (sp is short for split) is specific to vim. These days it's a pretty safe bet that you if you're on a Linux system you are running vim, but it's worth noting that if you find yourself on a system with vanilla vi (FreeBSD, Solaris) that this won't work.
–
larsksJun 16 '11 at 2:39

The keystrokes ctrl-w s will split the screen horizontally, and ctrl-w ctrl-w will switch your focus back and forth. You can widen the current viewport with ctrl-w +, or make is smaller with (any guess?) ctrl-w -.

You can do vertical splits with ctrl-w v, and you can split a split to make fairly complex sets of viweports inside the vim screen.